1. Field of the Invention
The present invention relates to a variable resistor suitable for use in, for example, detecting a tilt angle of a seat in an automobile.
2. Description of the Related Art
A related variable resistor will be described with reference to FIGS. 9 and 10. FIG. 9 is a vertical sectional view of the main portion of the related variable resistor. FIG. 10 is a transverse sectional view of the main portion of the related variable resistor. In the structure of the related variable resistor shown in FIGS. 9 and 10, a resistance element (not shown) and a resistance substrate 53 to which a terminal 52 is mounted are disposed in a recess of an exterior case 51 formed of an insulating material.
A rotor 55 having a sliding element 54 which slidably contacts the resistance element is mounted in the recess of the exterior case 51. Although not shown, a stopper is disposed between the rotor 55 and the exterior case 51 for stopping the rotation of the rotor 55.
An inner toothed portion 51a is formed on the inner peripheral surface defining the recess of the exterior case 51, and a groove 55a for accommodating a planetary disc 56 formed of rubber is formed in the rotor 55. The circular outer peripheral surface of the planetary disc 56 comes into resilient contact with the inner toothed portion 51a. 
An operating member 57 having a cross-shaped driver groove 57a and an externally toothed gear 57b is rotatably mounted to the top portion of the exterior case 51. The externally toothed gear 57b of the operating member 57 is positioned in the groove 55a and resiliently contacts the circular outer peripheral surface of the planetary disc 56, and pushes the planetary disc 56 against the inner toothed portion 51a. This causes the inner toothed portion 51a, the planetary disc 56, and the externally toothed gear 57b to form a reduction rotation transmission mechanism based on a planetary gear mechanism, and a clutch mechanism.
The resistance substrate 53 is secured to the lower portion of the exterior case 51 by filling the lower portion of the exterior case 51 with resin. Accordingly, the related variable resistor is formed.
In the related variable resistor having the aforementioned structure, when the operating member 57 is rotated in the direction of arrow A1 with a driver, the externally toothed gear 57b also rotates in the direction of arrow A1. Therefore, the planetary disc 56 rotates in the direction of arrow A2 by friction between the externally toothed gear 57b and the inner toothed portion 51a. 
When the planetary disc 56 rotates in the direction of arrow A2, the rotor 55 is pushed and rotates in the direction of arrow A3. As a result, the sliding element 54 comes into sliding contact with the resistance element, causing the resistance to vary. Continuing the rotation of the operating member 57 causes the rotor 55 to bump into the stopper at the exterior case 51, and to stop rotating. Further continuing the rotation of the operating member 57 causes a slip to occur between the externally toothed gear 57b and the planetary disc 56. (Refer to, for example, Japanese Unexamined Patent Application Publication No. 5-21214.)
However, the related variable resistor comprises a clutch mechanism using the rubber planetary disc 56. Therefore, when the rotation of the operating member 57 is continued while the rotation of the rotor 55 is stopped, the rotation of the externally toothed gear 57b in resilient contact with the planetary disc 56 causes considerable wearing of the planetary disc 56. As a result, in addition to not being possible to maintain the clutch mechanism and the reduction rotation transmission mechanism over a long period of time, the resistance cannot be finely because the reduction ratio in the planetary gear mechanism using one planetary disc 56 is small.
Since the related variable resistor comprises a clutch mechanism using the rubber planetary disc 56, it has the problem that it is not possible to maintain the clutch mechanism and the reduction rotation transmission mechanism over a long period of time because when the rotation of the operating member 57 is continued while the rotation of the rotor 55 is stopped, the rotation of the externally toothed gear 57b in resilient contact with the planetary disc 56 causes considerable wearing of the planetary disc 56.